Five sandstones, one shale, one limestone, and one marble in Japan were deformed experimentally for a study of the intermediate principal stress effect with a servo-controlled Mogi-type true triaxial compressional apparatus which allow us to control the three principal stresses, independently.
The intermediate principal stress changes the strength of rocks up to 30% but affects little on the residual strength. It was found that the increase of the intermediate principal stress induces the transition from the ductile to the brittle fracturing or the increase of brittleness with reduction of the plastic component of volumetric strain. It was newly observed that the effect of the intermediate principal stress on the strength increased with increase of the minimum principal stress, and that ultimate strength decreased with increase of the intermediate principal stress beyond a certain value.